The present invention relates to ferritic-austenitic two-phase stainless steel which is excellent particularly in thermal fatigue resistance and corrosion fatigue resistance and useful, for example, as a material for suction rolls for use in paper machines, and also to suction roll shell members prepared from the stainless steel.
The suction roll of the paper machine is a perforated roll for use in removing water from wet paper in the form of a web, and the shell portion of the roll is in the form of a hollow cylinder formed with a multiplicity of pores through which the water to be separated from the wet paper, i.e., so-called xe2x80x9cwhite water (strongly acidic corrosive liquid containing Clxe2x88x92, SO42xe2x88x92, etc.)xe2x80x9d is removed by suction. These pores are termed xe2x80x9csuction holesxe2x80x9d. The roll has as many as hundreds of thousands of suction holes, and the opening area ratio of the roll corresponds to about 20 to about 50% of the circumferential area of the roll shell portion.
The suction roll is not only exposed to the severe corrosive environment described but also subjected over the surface of its shell portion to a high pressure (nipping pressure) by a press roll for expressing water from the wet paper, and therefore has the problem of being liable to crack owing to corrosion fatigue. This problem has been handled by improving the material with use of two-phase stainless steel as a base, especially by giving enhanced corrosion fatigue strength to the material.
With increases in the papermaking speed in recent years, use of suction rolls in actual machines has encountered another problem anew. The heat of friction between the seals of the suction box and the roll shell portion in contact therewith raises the temperature of the inner surface of the roll shell portion, for example, to 300 to 350xc2x0 C., rendering the roll shell portion susceptible to cracking due to thermal fatigue caused by heat cycles.
With reference to FIG. 1, indicated at 1 is the shell portion of a suction roll, at 2 a press roll, and at 3 a suction box disposed in the interior of the shell portion of the suction roll. The roll 1 is formed with a multiplicity of suction holes 11. The suction box 3 bears on the inner peripheral surface of the roll shell portion 1, with seals 31 of phenolic resin or graphite interposed therebetween. Wet paper 5 is held by felt 4 and passes between the suction roll 1 and the press roll 2, as timed with the peripheral speed of the rolls. The water expressed from the paper is removed by the suction of the suction box 3 through the suction holes 11.
The heat generated by the friction of the seals 31 described is attributable mainly to maintenance problems such as an insufficient supply of lubricating water to the seals and excessive pressure applied to the seals against the roll inner peripheral surface. However, increases in the papermaking speed present difficulty in ensuring perfect maintenance for preventing these problems, entailing an increased likelihood of greater friction of the seals. It is therefore demanded to provide a novel roll material which is adapted to obviate the cracking of the suction roll and the shortening of roll life due to thermal fatigue.
To meet the demand, the present invention provides a two-phase stainless steel which has improved thermal fatigue resistance, realizes savings in the quantities of expensive and scarce elements such as Cr, Mo and Ni so as to be more economical, and has satisfactory workability with drills when used for making suction rolls.
The present invention provides a ferritic-austenitic two-phase stainless steel consisting essentially of, in wt. %, over 0% to not more than 0.05% of C, 0.1 to 2.0% of Si, 0.1 to 2.0% of Mn, 20.0 to 23.0% of Cr, 3.0 to 3.9% of Ni, 0.5 to 1.4% of Mo, over 0% to not more than 2.0% of Cu, 0.05 to 0.2% of N and the balance substantially Fe, Cr, Mo and N being within the range defined by the following expression [i]:
xe2x80x83Cr+3.3xc3x97Mo+16xc3x97Nxe2x89xa628%xe2x80x83xe2x80x83[i]
the metal structure of the stainless steel being 45 to 80% in the area ratio xcex1% of a ferritic phase therein, Cr and N further being within the range defined by the following expression [ii]:
0.2xc3x97(Cr/N)+25xe2x89xa6xcex1xe2x80x83xe2x80x83[ii]
When desired, at least one element can be incorporated into the ferritic-austenitic two-phase stainless steel of the invention, the element being selected from the group consisting of over 0% to not more than 0.5% of Ti, over 0% to not more than 0.5% of Nb, over 0% to not more than 1.0% of V, over 0% to not more than 0.5% of Al, over 0% to not more than 0.5% of Zr, over 0% to not more than 0.5% of B, over 0% to not more than 0.2% of a rare-earth element, over 0% to not more than 1.0% of Co, over 0% to not more than 1.0% of Ta and over 0% to not more than 1.0% of Bi.
Cr, Mo and like element present in two-phase stainless steels exert a great influence on the corrosion resistance of the steel, and it is said that the corrosion resistance increases generally as the content of such an element increases. However, these elements are expensive and scarce and are each a ferrite former, so that if they are used in an increased amount, there arises a need to add an increased amount of Ni which is an austenite former and is similarly a scarce element, in view of the component balance. On the other hand, increases in the quantities of these elements entail a reduction in the workability by drilling.
The two-phase stainless steel of the present invention is improved in thermal fatigue resistance and corrosion fatigue resistance characteristics and is satisfactory in workability with drills while ensuring savings in the quantities of these scarce elements to be used.